Aircraft wings are generally equipped with aerodynamic control elements including control surfaces such as ailerons, elevators, rudders and spoilers, as well as high lift devices such as flaps and slats. These aerodynamic control elements are independently operable and controllable so as to define the lifting force produced by the respective wing and the respective aerodynamic control element, and to control the drag forces being generated. Thereby, appropriate operation of the control elements makes it possible to adjust the flight attitude and to steer the aircraft along an ascending or descending flight path, as well as with a rolling motion and/or yawing motion. The rolling motion is controlled by asymmetric deflection of respective suitable control elements on the left and right wings respectively, while the ascending or descending flight attitude is controlled by symmetric deflection of respective suitable control elements on the two wings. It is known to deploy the spoilers in order to reduce lift and increase drag and thereby achieve a steeper descent of the aircraft by symmetric deflection of the spoilers on both wings in an airbrake mode. It is further known to deploy the spoilers to control or enhance a rolling motion by asymmetric deflection of the spoilers on both wings.
During flight, an aircraft generates wake turbulence that trails behind the aircraft. Such wake turbulence mainly comprises wake vortices that are initiated at the trailing edges and the wing tips of the wings, and spiral or helically trail behind the wing as a direct consequence of the lift generation by the wing. These spiraling vortices trailing behind in the wake of an aircraft (the so-called “leader aircraft”) may interfere with or even become hazardous to any other aircraft (the so-called “follower aircraft”) that follows too close behind the leader aircraft. For this reason, applicable flight regulations specify longitudinal Wake Turbulence Separation Standards that must be maintained between successive aircraft approaching an airport terminal area. The existing standards are based on the maximum take-off weight category of the leader aircraft and the follower aircraft under consideration. These standards are usually specified in nautical miles (NM), and range from 3 NM for a light follower aircraft trailing behind a light leader aircraft, up to 6 NM for a light follower aircraft in trail of a heavy aircraft. The required application of Wake Turbulence Separation Standards at airports limits the number of aircraft that can land on a given runway during a given period of time and thus limits total flight throughput capacity of the airport.
It has been proposed in the prior art, to install warning systems to warn aircraft of wake turbulence conditions in the vicinity of airports, as well as guidance systems for guiding aircraft with appropriate wake turbulence separation during the approach to airports. For example, U.S. Pat. No. 6,177,888 discloses a wake turbulence warning and caution system and method, in which the wake trajectory of a nearby aircraft is calculated, and warning functions are implemented aboard the protected aircraft that follows or will intersect the trailing wake of the leader aircraft. PCT Publication WO 00/010064 discloses an aircraft approach method and instrument landing system for carrying out such a method. Each aircraft approaching an airport is guided along an individual trajectory. Such individualized descent and landing trajectories, which each have an individual descent slope, provide a higher landing rate of successive aircraft while avoiding problems of wake turbulence that require sufficient separation distance between successive landing aircraft.
It has further been considered in the prior art, and has been the subject of research, to provide aerodynamic control elements and methods of operation thereof, to influence the creation as well as the attenuation and decay of wake vortices. Particularly, it has been considered in the prior art whether aircraft spoilers would be effective for reducing such wake vortices. For example, it has been considered to use existing spoilers to generate turbulence that might reduce the hazard of vortices in the wake, but it was determined that the effect of spoilers, control of engine thrust, and the use of other turbulence injection devices was mainly a reduction of the rotary velocity near the core of wake vortices, and did not seem to sufficiently accelerate the decay of the vortices (V. Rossow, “Lift-Generated Wakes of Subsonic Transport Aircraft”, Progress in Aerospace Science 35, 1999, pages 507-660). Further, the symmetrical deflection of various combinations of spoiler segments on an aircraft, with a constant deflection angle of 45°, found that existing flight spoilers with such a large constant deflection angle may be capable of attenuating trailing vortices (D. R. Croom, “Evaluation of Flight Spoilers for Vortex Alleviation”, Journal of Aircraft, August 1977, pages 823-825). Still further, tests were conducted on an aircraft with different flap settings and a fixed or constant spoiler extension of 41°, to conclude that the spoilers produced vortices with large cores, and that the deployment of flaps and spoilers with the stated large constant extension angle enhanced the decay of the vortex peak tangential velocity in the near field wake behind the aircraft, while the aircraft attitude, glide slope and deployment of landing gear had little influence on the vortex decay process (J. Hallock et al., “Ground-Based Measurements of the Wake Vortex Characteristics of a B-747 Aircraft in Various Configurations”, AIAA 15th Aerospace Sciences Meeting, Paper 77-9, 1977, Pages 1 to 8).
The prior art has thus considered the use of statically deflected spoilers for vortex attenuation purposes. On the other hand, due to the reduction of lift and increase of drag, it would be highly uneconomical to fly with spoilers symmetrically deployed at such a large constant deflection angle. Even during steep approach, such deflection of spoilers is only appropriate temporarily for glide slope capture or for intermittent airbrake mode. Moreover, the spoiler settings for airbrake mode are not designed to be efficient for wake alleviation. Still further, the approach and landing phase of flight requires asymmetric use of spoilers for lateral control, but such asymmetric spoiler deflection has not been shown to be effective on wake alleviation, and is not compatible with wake alleviation because such spoiler deflection for lateral control is applied only intermittently and temporarily, while spoiler deflection for vortex alleviation would need to be applied constantly. Even more importantly, the large spoiler deflections (more than 40°) that have been tested for wake alleviation in the prior art, cause a significant degradation of aircraft performance due to decreased lift and increased drag, and as such are not applicable for wake alleviation during standard approach and landing phase operation of an aircraft.